what is the VTE treatment dose and monitoring recommended for enoxaparin in patients with extremely low body weight

What is the VTE treatment dose and monitoring recommended for enoxaparin in patients with extremely low body weight < 50 kg and standard renal function?

Comment by InpharmD Researcher

Evidence specifically evaluating enoxaparin for treatment of venous thromboembolism (VTE) in patients weighing <50 kg is limited. Available expert recommendations generally support dosing based on total body weight, with some specifically recommending enoxaparin 1 mg/kg subcutaneously every 12 hours; in patients weighing <40 kg, unfractionated heparin has been suggested because of limited data for low-molecular-weight heparins. Small observational studies in patients weighing <45 to ≤50 kg have reported variable anti-factor Xa concentrations with weight-based treatment dosing, while registry data suggest a higher incidence of bleeding in patients <50 kg, although differences in dosing and patient characteristics limit interpretation. Routine peak anti-factor Xa monitoring is not generally recommended, but individualized monitoring may be considered in low-body-weight patients because of variable anti-factor Xa concentrations; when measured, peak levels have typically been obtained 3 to 5 hours after a dose.

Background

A 2016 guidance article initiated by the Anticoagulation Forum noted that evidence regarding low-molecular-weight heparin (LMWH) dosing in patients with low body weight is limited, with the lowest body weight reported in an enoxaparin venous thromboembolism (VTE) clinical trial being 44 kg and patients weighing <40 kg excluded from a major dalteparin VTE trial. In a registry of 7,962 patients receiving LMWH for acute VTE, 161 patients weighed <50 kg; compared with patients weighing 50-100 kg, patients <50 kg had higher rates of major bleeding (3% vs 1.3%) and minor bleeding (5.3% vs 2.5%; odds ratio [OR] 2.2; 95% confidence interval [CI] 1.2 to 4), although 54% of patients in the <50-kg group received doses >200 international units/kg/day, and recurrent VTE rates were similar between the groups. Based on the available evidence and expert consensus, the guidance recommends that LMWH treatment dosing be based on total body weight, including in underweight patients, and that dose capping be avoided; for patients weighing <40 kg, unfractionated heparin may be more appropriate given the limited clinical data. The guidance also does not recommend routine peak anti-factor Xa monitoring during LMWH therapy, noting that peak anti-factor Xa concentrations have not been demonstrated to correlate with clinical effectiveness. [1]

A 2018 literature review found that no clinical trials specifically evaluated enoxaparin for VTE treatment in patients with low body weight, limiting the evidence for dose adjustment in this population. In the RIETE observational registry (see Table 1), patients weighing <50 kg had a higher overall incidence of bleeding than those weighing 50-100 kg (OR 2.2; 95% CI 1.2 to 4.0), although low body weight was not significantly associated with major bleeding on univariate analysis. The low-weight group also had potential confounding factors, including greater nonsteroidal anti-inflammatory drug use and a higher prevalence of underlying conditions such as cancer and immobility. Limited pharmacokinetic data suggest that anti-factor Xa exposure may increase as body weight decreases. Two patients weighing 40-42 kg who received enoxaparin 1 mg/kg twice daily had a mean anti-factor Xa concentration of 1.4 IU/mL, above the study target of 0.5-1.1 IU/mL; however, the sample was too small for statistical analysis or conclusions regarding clinical outcomes. [2]

Despite these findings, the authors concluded that the available evidence does not establish an alternative treatment dose for low-weight patients and recommended using total body weight with standard treatment dosing of enoxaparin 1 mg/kg subcutaneously every 12 hours. For monitoring, the review recommends assessment of renal function, signs and symptoms of bleeding, and signs and symptoms of recurrent VTE. There is no clear consensus supporting routine anti-factor Xa monitoring, as anti-factor Xa concentrations are primarily surrogate markers and have not been strongly associated with thrombosis or bleeding. However, anti-factor Xa monitoring may be considered on an individualized basis in low-weight patients, If obtained with twice-daily treatment dosing, the authors cite an expert-opinion peak anti-factor Xa range of 0.5-1.0 IU/mL, while emphasizing that the optimal therapeutic range and clinical value of monitoring remain uncertain. [2]

Literature Review

A search of the published medical literature revealed 3 studies investigating the researchable question:

what is the VTE treatment dose and monitoring recommended for enoxaparin in patients with extremely low body weight < 50 kg and standard renal function?

Level of evidence

C - Multiple studies with limitations or conflicting results Read more→

Please see Tables 1-3 for your response.

The influence of extreme body weight on clinical outcome of patients with venous thromboembolism: findings from a prospective registry (RIETE)
Design	Prospective registry study N= 8845
Objective	To compare the clinical characteristics, treatment options, and clinical outcomes during the first 15 days of therapy of three different body-weight groups of VTE patients included in the RIETE registry
Study Groups	<50 kg (n= 169) 50-100 kg (n= 8382) >100 kg (n= 294)
Inclusion Criteria	Patients with objectively confirmed, symptomatic acute VTE enrolled in the RIETE registry
Exclusion Criteria	Patients participating in a therapeutic clinical trial or not available for a 3-month follow-up
Methods	Patients were classified into three weight groups: <50 kg, 50-100 kg, and >100 kg. Clinical characteristics, underlying conditions, risk factors for VTE, and treatment options were compared. The rate of VTE recurrences, bleeding complications, and death at the end of the 15-day study period were calculated. Data were collected from 94 Spanish hospitals participating in RIETE
Duration	Data collection until August 2004
Outcome Measures	Rate of VTE recurrences, bleeding complications, and death during the first 15 days
Baseline Characteristics		<50 kg (n= 169)	50-100 kg (n= 8382)	>100 kg (n= 294)
	Gender (males)	30 (18%)	4202 (50%)	188 (64%)
	Age (>65 years)	110 (65%)	5308 (63%)	86 (29%)
	Chronic lung disease	31 (12%)	944 (11%)	38 (13%)
	Cancer	53 (21%)	1673 (20%)	32 (11%)
	Immobility	66 (39%)	2241 (27%)	70 (24%)
Results		<50 kg (n= 169)	50-100 kg (n= 8382)	>100 kg (n= 294)
	Major bleeding	5 (3.0%)	110 (1.3%)	3 (1.0%)
	Minor bleeding	9 (5.3%)	218 (2.6%)	4 (1.4%)
	Total bleeding	14 (8.3%)	328 (3.9%)	7 (2.4%)
	Fatal PE	1 (0.6%)	121 (1.4%)	1 (0.3%)
	Recurrent VTE	2 (1.2%)	86 (1.0%)	2 (0.7%)
	Overall death	13 (7.7%)	255 (3.0%)	2 (0.7%)
	Among patients weighing <50 kg, most LMWH (specific agent unspecified), and although their mean daily LMWH dose was lower than that of patients weighing 50–100 kg, 54% received >200 IU/kg/day compared with 21% of the 50–100 kg group. Patients <50 kg had higher overall bleeding rates, including major bleeding in 3.0% vs 1.3% and minor bleeding in 5.3% vs 2.6%, corresponding to an odds ratio of 2.2 (95% CI, 1.2–4.0). Of the five major bleeding events in patients <50 kg, three occurred in those receiving >200 IU/kg/day of LMWH. Recurrent venous thromboembolism occurred in 1.2% of patients <50 kg, and this group also had higher mortality; however, they more frequently had severe underlying conditions, including cancer and immobility, which may have contributed to these outcomes.
Adverse Events	Patients weighing <50 kg had a significantly higher rate of bleeding complications. No significant differences in major bleeding or VTE recurrences among patients weighing <50, 50-100, or >100 kg.
Study Author Conclusions	Patients with VTE weighing <50 kg have a significantly higher rate of bleeding complications. The clinical outcome of patients weighing over 100 kg was not significantly different from that in patients weighing 50-100 kg.
Critique	The study provides valuable insights into the impact of extreme body weight on VTE outcomes, but its observational design may introduce bias. The lack of randomization and potential variability in heparin dosing are limitations. However, the large sample size and real-world data offer important information for clinical practice.

Table 1 References:
[3] Barba R, Marco J, Martn-Alvarez H, et al. The influence of extreme body weight on clinical outcome of patients with venous thromboembolism: findings from a prospective registry (RIETE). J Thromb Haemost. 2005;3(5):856-862. doi:10.1111/j.1538-7836.2005.01304.x

The use of anti-factor Xa monitoring in a selection of patients receiving enoxaparin at a large academic medical center
Design	Retrospective, non-interventional, medical chart review N= 273
Objective	To characterize the use of enoxaparin in patients over 150 kg, under 45 kg, and those with renal dysfunction, and to describe the rate of anti-factor Xa monitoring in these patients
Study Groups	<45 kg (n= 96) >150 kg (n= 111) Renal dysfunction (n= 66)
Inclusion Criteria	Patients ≥18 years of age receiving therapeutic enoxaparin with a diagnosis of pulmonary embolism, deep vein thrombosis, venous thromboembolism, or atrial fibrillation, categorized into weight >150 kg, weight <45 kg, or renal dysfunction (CrCl ≤30 mL/min)
Exclusion Criteria	Patients receiving prophylactic enoxaparin
Methods	Data was collected from inpatient admissions between January 2010 and October 2015. Patients were categorized into three arms based on weight and renal function. Anti-factor Xa levels were drawn and analyzed, focusing on peak levels drawn 3-5 hours post-dose. The study assessed the rate of monitoring and dose adjustments based on anti-factor Xa levels
Duration	January 2010 to October 2015
Outcome Measures	Primary: Rate of LMWH anti-factor Xa monitoring Secondary: Doses achieving therapeutic levels, timing and assessment of LMWH anti-factor Xa levels, rate of major bleeding events
Baseline Characteristics		<45 kg (n= 96)	>150 kg (n= 111)	Renal dysfunction (n= 66)
	Age	54 (37–54)	49 (42–59)	81.5 (75.8–86.3)
	Gender (% female)	93.8%	50.5%	69.7%
	Weight, kg	41.7 (39.9–43.5)	159.3 (152.9–170.2)	68.1 (61.2–81.9)
	BMI, kg/m2	16.8 (15.5–18.0)	52.4 (47.0–57.5)	25.1 (21.8–29.2)
	Baseline SrCr, mg/dL	0.5 (0.4–0.7)	0.9 (0.7–1.0)	1.7 (1.3–2.0)
	Baseline calculated CrCl, mL/min	76.9 (54.1–103.8)	140.9 (116.1–182.3)	27.8 (25.0–30.2)
Results	Among 96 patients weighing <45 kg, 28 (29%) underwent LMWH anti-factor Xa monitoring, yielding 42 levels, of which only 24 (57.1%) were appropriately timed peak levels. Among peak measurements, the median anti-factor Xa level was 0.47 U/mL (0.31-0.58 U/mL), which was subtherapeutic, and 60.9% of patients had subtherapeutic levels; the remaining patients had therapeutic levels. The median enoxaparin dose preceding the evaluated peak levels was 1.01 mg/kg/dose (0.93-1.21 mg/kg/dose). One supratherapeutic level was observed, although it was not a peak measurement.
Adverse Events	Bleeding events were minimal, occurring in 8 patients, with no events meeting TIMI major criteria. Most bleeding events occurred in the <45 kg group.
Study Author Conclusions	Enoxaparin has unpredictable pharmacokinetics in high-risk populations such as those who are underweight, overweight, or have renal dysfunction. Anti-factor Xa monitoring may be necessary to ensure therapeutic levels and appropriate dosing.
Critique	The study highlights the need for anti-factor Xa monitoring in high-risk populations, but its retrospective nature and small sample size limit the generalizability of the findings. The low rate of monitoring and the lack of steady-state level evaluation are notable limitations

Table 2 References:
[4] Sacha GL, Greenlee KM, Ketz JM. The use of anti-factor Xa monitoring in a selection of patients receiving enoxaparin at a large academic medical center. J Thromb Thrombolysis. 2016;42(4):479-485. doi:10.1007/s11239-016-1384-x

Enoxaparin Dosing and AntiXa Monitoring in Specialty Populations: A Case Series of Renal-Impaired, Extremes of Body Weight, Pregnant, and Pediatric Patients
Design	Single-center, retrospective review N= 31
Objective	To evaluate the use of enoxaparin and antiXa monitoring in specialty populations, including renal-impaired, extremes of body weight, pregnant, and pediatric patients, to determine optimal dosing and monitoring strategies
Study Groups	Age ≤ 18 (n= 11) CrCl < 30 mL/min (n= 3) CrCl 30–50 mL/min (n= 5) ABW ≤ 50 kg (n= 7) ABW ≥ 120 kg (n= 2) Pregnant (n= 3)
Inclusion Criteria	Patients ≤ 18 years of age or > 18 years of age with criteria: pregnant, CrCl ≤ 50 ml/min, body weight ≤ 50 kg or ≥ 120 kg
Exclusion Criteria	Patients who received fewer than 3 enoxaparin doses, prophylactic-dosed enoxaparin, or no corresponding antiXa drawn at steady state
Methods	Retrospective review of patients receiving at least 3 consecutive doses of enoxaparin with peak antiXa level drawn at steady state. AntiXa levels were measured 3 to 5 hours post-dose. Data collected included demographics, dosing regimens, antiXa measurements, and adverse events
Duration	December 2012 to July 2014
Outcome Measures	Percentage of patients achieving therapeutic antiXa at first level drawn
Baseline Characteristics		Age ≤ 18 (n = 11)	CrCl < 30 mL/min (n = 3)	CrCl 30–50 mL/min (n = 5)	ABW ≤ 50 kg (n = 7)	ABW ≥ 120 kg (n = 2)	Pregnant (n = 3)
	Age (years), median (range)	0.5 (0–18)	83 (80–89)	85 (68–88)	80 (48–89)	63 (55–71)	35 (29–39)
	Sex (female)	6 (54%)	3 (100%)	5 (100%)	2 (29%)	1 (50%)	3 (100%)
	Baseline CrCl (mL/min), median (range)	122 (26–1008)	26 (24–29)	39 (30–49)	49 (32–184)	123 (68–177)	107 (100–122)
	Weight (kg), median (range)	23 (3–57)	48 (39–59)	51 (39–63)	46 (44–49)	128 (124–132)	73 (58–85)
Results	Among 1,552 screened encounters, 31 patients were included in the final analysis after excluding patients without appropriately timed steady-state anti-factor Xa concentrations and other eligibility criteria. In the low-body-weight subgroup (actual body weight ≤50 kg; n=7), 4 patients (57%) achieved a therapeutic anti-factor Xa concentration with the first measurement, while 3 (43%) were subtherapeutic and none were supratherapeutic; the institutional therapeutic range was 0.5-1.2 IU/mL for twice-daily dosing and 1-2 IU/mL for once-daily dosing. At day 7, the same proportions remained therapeutic (57%) and subtherapeutic (43%), and the mean time to reach the therapeutic range was 2 days (range, 1-3 days). Across the overall study population, the mean initial adult dose for twice-daily enoxaparin was 1.09 ± 0.16 mg/kg per dose. No major or minor bleeding events, thrombocytopenia, or suspected heparin-induced thrombocytopenia were reported, and no recurrent or progressive thrombosis was reported in the low-body-weight subgroup.
Adverse Events	No major or minor bleeding events, thrombocytopenia, or suspicion of HIT observed. One pediatric patient had suspicion of thrombosis progression.
Study Author Conclusions	Optimal dosing of enoxaparin in specialty populations remains uncertain. Monitoring antiXa levels may help guide dosing, especially in pediatric patients who may require higher initial doses to achieve therapeutic levels. Monitoring trough antiXa levels may be more beneficial in patients at risk of bioaccumulation.
Critique	The study is limited by its retrospective design, small sample size, and lack of prospective data. The findings are consistent with previous literature but highlight the challenges in monitoring and dosing enoxaparin in specialty populations. Further prospective studies are needed to establish optimal dosing and monitoring strategies.

Table 3 References:
[5] Ahuja T, Mousavi KM, Klejmont L, Desai S. Enoxaparin Dosing and AntiXa Monitoring in Specialty Populations: A Case Series of Renal-Impaired, Extremes of Body Weight, Pregnant, and Pediatric Patients. P T. 2018;43(10):609-614.